Almost all devices of modern entertainment electronics have visual display systems for displaying the operating state of the device or further information. If more complex relationships are to be represented, display modules based on liquid crystals (LCD) or on organic light-emitting diodes (OLED) are frequently used for the display. Such displays are used, for example, in digital cameras, portable minicomputers and mobile telephones.
In order to protect the display modules from possible damage through external mechanical action, such as, for example, shocks, such display systems conventionally have transparent protective windows which cover the outside of the display modules and thus reduce the risk of direct action on the module. Such protection is likewise necessary in the case of non-electronic visual display systems, for example in the case of mechanical displays such as, for example, timepieces or level indicators on storage vessels.
Polymer sheets (for example of polycarbonate (PC), polymethyl methacrylate (PMMA)) or glass sheets are conventionally used as the protective windows, wherein each of the two systems has advantages and disadvantages and is therefore to be chosen according to the specific application.
Although polymer sheets are inexpensive as well as easy to process and provide efficient protection from mechanical actions, they have the disadvantage that they are not usually scratch-resistant and therefore are easily damaged. This not only leads to an impairment of the esthetic impression of the display systems after only a short time but also results in a diminished view of the display region of the display modules. In addition, many common polymers have only limited resistance to ultraviolet light (UV light) or organic solvents.
Protective windows of glass, on the other hand, are inert towards organic solvents and, because of their high hardness, are also scratch-resistant, so that they impart a high-quality impression. However, because of the brittleness of this material, resulting from its hardness, glass has only limited suitability as protection against mechanical actions such as shock or impact because splintering brittle fracture of the glass sheet can occur even at low stresses. In addition to the only limited protective action, there is accordingly also the risk of injury due to the resulting splinters as well as the risk of damage to the display module by sharp-edged fragments.
Sheets of glass or other transparent or translucent materials are also used when optical functions, such as light refraction, focusing, attenuation or amplification, are to be performed. When such lenses are fitted into the mount, or the device body, the requirements are similar to those of the above-described windows. The problems are therefore comparable.
The fixing of protective display windows or optical lenses in the casings of electronic devices, in particular small portable devices such as mobile telephones and the like, which casings are conventionally made of plastics material or metal, is today carried out mainly by means of double-sided adhesive tapes. The person skilled in the art is therefore interested in suitable and ever better adhesives for double-sided adhesive tapes for the adhesive bonding of such cover glasses or lenses to mounts or casings. The profile of requirements for adhesives for such applications includes high push-out strength (that is to say the bond strength of the component in its mount) and, at the same time, high impact strength even at low temperatures. In addition, a high adhesive power even on non-polar substrates is frequently required, for example for better adhesive bonding to printed substrates; printed layers, for example, can thus have a low-energy surface. A certain “reworkability” even for permanently bonded substrates is additionally advantageous, that is to say the device can be disassembled within a short period of time after assembly or even after a prolonged time in such a manner that individual components can be recovered in a residue- and damage-free manner. High heat resistance is likewise frequently desirable.
EP 349 216 A1 describes that styrene block copolymers (SBC) can be added to polyacrylate pressure sensitive adhesives, which are produced in the form of a so-called UV syrup, in order to improve the low-temperature impact strength. Typical added amounts of the SBC are 5 parts to 35 parts for 95 parts to 65 parts of the acrylate component. An application as described in the introductory part of this specification, in particular taking into consideration the specific balance of requirements of push-out strength and ball-drop resistance, is not disclosed.
EP 352 901 B1 relates to pressure sensitive adhesives comprising from 65 to 95 parts of a UV-polymerized polyacrylate and from 35 to 5 parts of a synthetic rubber; EP 437 068 B1 discloses cellular pressure sensitive adhesive membranes based on polyacrylate/SBC blends. Improved low-temperature impact strength and adhesion to paints is discussed. These specifications are not directed to the application discussed herein.
WO 2000/006637 A1 teaches foamed adhesive layers. Blends consisting of acrylates and SBC are mentioned, but likewise for different fields of application.
WO 2012/062589 teaches examples of similar adhesive bonding applications as in the present specification, but without mentioning corresponding adhesives.
Adhesives that are suitable in particular for the adhesive bonding of windows or lenses in mounts or casings are therefore sought. A balanced combination of high push-out strength and impact strength even at low temperatures is desired. High adhesive powers on substrates of low polarity, good reworkability and heat resistance are advantageous.